Two-component polyurethane (PU) coating materials are used for top coating in the automobile industry on account of their high resistance to environmental effects, particularly acid rain, by comparison with conventional coating systems crosslinked with amino resin (W. Wieczorrek in: Stoye/Freitag, Lackharze [resins for coatings], p. 215 if, C. Hanser-Verlag, 1996; J. W. Holubka et al., J. Coat. Tech. Vol. 72, No. 901, p. 77, 2000). Used here in general are OH-functional poly(meth)acrylate resins and polyisocyanates based on hexamethylene diisocyanate (HDI). The use of other diisocyanates or other diisocyanate derivatives is likewise possible.
Work has been ongoing on the development of new automotive clearcoat materials for many years. In view of the rising quality requirements in the automobile industry, work is ongoing in particular into improving the resistance toward environmental influences, and high mechanical stability, particularly the scratch resistance.
One possibility of improving the scratch resistance of two-component polyurethane coatings is to introduce trialkoxysilane groups into the coating components. These silane components are capable of raising the stability of polyurethane coatings by crosslinking and formation of siloxane networks. This principle is taught by EP 1 273 640. This publication describes two-component (2K) coating materials comprising a polyol component and a crosslinker component, consisting of aliphatic and/or cycloaliphatic polyisocyanates or the polyisocyanates derived from them by polymerization, allophanatization, biruetization or urethanization, where 0.1 to 95 mol % of the originally free isocyanate groups present have undergone reaction with bisalkoxysilylamine. These coating materials can be used for producing clearcoat or topcoat finishes in the motor vehicle segment, and, after they have cured completely, exhibit good scratch resistance in conjunction with good resistance toward environmental effects. Sandability and polishability and also the visual impression given by the resultant coatings, however, are still capable of further improvement.
The principle of polyurethane coatings which comprise polyisocyanate compounds with proportional reaction of the isocyanate groups to form silane groups has been pursued continuously in recent years—compare, among others, patent applications WO 2007/033786, WO 2008/74489, WO 2009/077181, US 2011/0269897, WO 2012/168079 and WO 2014/086530. These texts describe various alkoxysilane-containing polyisocyanates as a component for the crosslinking with polyols. For the reaction with polyisocyanates, various monoaminosilane, bisaminosilane and mercaptosilane building blocks and also mixtures thereof are used. Besides the use of various silane building blocks, new catalysts are also described for the improved crosslinking of the silane structures.
A second approach relates to improving the scratch resistance of two-component polyurethane coatings by adding isocyanate-free, silane-terminated prepolymers. Approaches of this kind are described for example in patent applications EP 2676982 and EP 2735578.
In view of the continually increasing quality demands imposed on automotive clearcoats, there continues to be a need for new building blocks for the production of such coating formulations.
Very generally, silane-modified compounds of the type claimed here are those compounds which contain silane groups having hydrolyzable radicals, and whose polymer backbone is not constructed substantially of O—Si—O—Si chains, as is the case with the silicones, but is instead constructed from C—C chains which, in the majority of cases, are interrupted by heteroatoms and additionally contain urethane, ether, ester, urea, amide and other structural units. On exposure to moisture and/or under the influence of suitable catalysts, the radicals on the silane groups—usually, for example, acetate or alkoxy groups—are hydrolyzed, forming reactive silanols which subsequently condense and cure to form a high molecular mass network, eliminating water, alcohol or acetic acid as they do so.
Compositions which comprise silane-modified compounds are notable for qualities including a high level of adhesion to any of a very wide variety of substrates, without costly and inconvenient pretreatment (no primer necessary). The reason for this is that normally OH groups are present on inorganic substrate surfaces, and are able to react with the reactive silanols of the polymer that are formed on exposure to moisture.
The silane-modified polyurethanes and polyureas that are presently available commercially on the market are based on a high molecular mass backbone which is generated (i) by reaction of NCO-containing prepolymers with aminosilanes, or (ii) by reaction of OH-terminated prepolymers, such as polyethers, polyurethanes or polyesters, for example, with NCO-functional silanes, or (iii) by reaction of NCO-containing prepolymers with mercaptosilanes, as shown in the formula scheme below:

Furthermore—as already mentioned—the hardness of the resulting coating as the end product after the silane crosslinking is very important for auto paints. In the case of silane-modified polyureas, the end products normally have a high hardness; possibly, however, the coating becomes so rigid and highly crosslinked that cracks may appear. In contrast, silane-modified polyurethanes provide end products that are softer after curing. Nevertheless, the synthesis of silane-modified polyurethanes with a high silane content is difficult economically to implement, owing to the relatively expensive NCO-functionalized silane precursors.